This invention relates generally to an improved radiation detector and more specifically to a radiation detector with self-limiting amplification properties.
Semiconductor solid-state radiation detectors have long been used in nuclear medicine, science and industry. Such devices are useful in detecting radiation such as X-rays, gamma rays, electron beams, or neutron beams.
Solid state detectors generally operate utilizing a process known as ionization. During ionization, radiation that impinges into the detector will cause valance electrons and holes (more generally referred to as charge carriers) to be freed in the detector. The number of freed charge carriers will depend on the energy of the impinging radiation, and therefore this energy can be measured. The freed charge carriers create a small electrical signal indicative of when and where an ionization event has occurred within the detector, and the energy thereof.
Prior art solid state detectors generally consist of two types, both of which suffer from inherent operational problems. In widely-accepted detectors of the first type, the small electrical signals produced by the freed charge carriers are sensed without being amplified within the detector. Because these small signals are generally very weak, they must be subsequently amplified in order to produce an electrical signal suitable for subsequent processing. However, subsequent amplification introduces additional noise, therefore hampering the detection of relatively low-energy ionization events.
In detectors of the second type, the small signals produced by the freed charge carriers are amplified within the detector itself through a process called avalanche multiplication. In avalanche multiplication, an electron (or hole) freed from a first atom is accelerated within the detector material (e.g., a crystalline silicon lattice) by an electric field produced by an external electrical bias applied to the detector material. If the electron gains enough energy by this acceleration, it can then impinge upon a second atom and free an electron from it, thus producing two freed electrons (and holes). This process continues in an exponential fashion until many freed charge carriers are produced. Thus, through avalanche multiplication, the initial small signal produced by the initial ionization event is intrinsically amplified within the detector and produces a stronger signal better suited for use in subsequent signal processing. While one of ordinary skill will understand the basic principles of avalanche multiplication as it applies to radiation detectors, the reader is referred to S. M. Sze, "Physics of Semiconductor Devices," John Wiley & Sons, pp. 99-108 (1981), which is incorporated herein in its entirety, for further clarification. Because avalanche multiplication is a fast, low noise method for amplifying a small signal produced by an ionization event, detectors using avalanche multiplication may be superior for certain applications.
However, intrinsic amplification by avalanche multiplication is not without its drawbacks. The exponential generation of charge carriers by avalanche multiplication makes it very difficult to control. This presents a serious drawback in that: 1) due to the stochasticity of the process, it is difficult to know to what extent an initial small signal will be subsequently amplified using avalanche multiplication, and 2) the detector may be damaged if the avalanche multiplication cannot be controlled. Moreover, traditional p-i-n detectors (i.e., wherein an intrinsic, undoped semiconductor is situated between a p-doped semiconductor and an n-doped semiconductor) using avalanche multiplication require undesireably high biasing voltages (e.g., hundreds of Volts). Moreover, such devices are difficult to fabricate in a manner that will provide a suitably high spatial resolution of detected radiation.
The disclosed detector improves upon the prior art in a manner which alleviates the foregoing problems by presenting a novel detector design which uses a self-limiting avalanche multiplication mode of operation.